APPLIED PHYSICS LETTERS
VOLUME 72, NUMBER 15
13 APRIL 1998
Significant reduction of cathodoluminescent degradation in sulfide-based
phosphors
J. M. Fitz-Gerald, T. A. Trottier, R. K. Singh,a) and P. H. Holloway
Department of Materials Science and Engineering, University of Florida, Gainesville,
Florida 32611-6400
~Received 24 November 1997; accepted for publication 12 February 1998!
The degradation of cathodoluminescent ~CL! brightness under prolonged electron-beam excitation
of phosphors has been identified as one of the outstanding critical issues for flat-panel field-emission
displays. In this letter, we have demonstrated that a TaSi2 coating on Y2O2S:Eu31 phosphors
substantially inhibits the cathodoluminescent degradation characteristics without reducing its
efficiency. The coating was deposited by pulsed laser deposition of TaSi2 targets onto a fluidized
bed containing phosphor particles. Cathodoluminescent degradation experiments conducted at 2
keV and at 150 mA/cm2, showed that the CL brightness decreased by more than 50% after a
Coulomb load of 15 C/cm2 on the uncoated material. In contrast, the TaSi2-coated phosphor
powders showed much less degradation, with CL brightness only decreasing by approximately 12%
after electron irradiation with the same dose. © 1998 American Institute of Physics.
@S0003-6951~98!01715-X#
Sulfide-based phosphors such as Y2O2S:Eu31, ZnS:Ag,
and ZnS:Cu are potential phosphor materials for fieldemission display applications.1–5 It is well known that the
sulfur-containing phosphors exhibit the highest luminous efficiencies of all the currently available industrial phosphors.
However, one of the outstanding problems in the use of the
sulfide-based phosphors is the cathodoluminescent ~CL!
degradation.4–8 Under standard operating conditions ~low accelerating voltage ,7 keV, high current density
.5 mA/cm2, vacuum 131025 Torr, Coulomb load
515 C/cm2!, the sulfur-based phosphors can degrade more
than 50% from their original brightness. The CL degradation
is related to the total charge impressed upon the phosphor
screen. Although many different mechanisms have been reported for the degradation of phosphors, many relate to similar surface phenomena. Recent studies in CL degradation
have centered around sulfur-containing phosphors.7,8 Studies
on ZnS:Ag have shown that during electron-beam aging,
both carbon and sulfur are depleted from the near-surface
regions of the phosphor with concomitant increase in the O
and Zn surface concentration.7 The surface region of the ZnS
phosphor is converted to a sulfur-depleted, oxygen-rich compound, such as ZnO or ZnSO4. 7 Due to this nonluminescent
‘‘dead layer,’’ which can range up to 0.4 mm thick, the
cathodoluminescent efficiency is dominated by the power
loss of the electron beam in the nonluminescent layer.
One possibility for slowing the degradation rate is to
coat the surface of the phosphors with a material that inhibits
CL loss. In order to be commercially viable, the coating must
not be detrimental to the handling qualities, brightness, and
chromaticity of the phosphor and should be thin enough to
be transparent at low energies. Several coatings have been
investigated by using wet-based deposition techniques.8 The
thickness, composition, and coverage depend on the chemistry of the wet process.
Some recent studies on oxide and phosphate coatings depos-
ited by precipitation methods have not shown significant
change in the CL degradation properties of Y2O2S:Eu31
phosphor. In this letter, we report the use of TaSi2 coating to
significantly reduce the CL degradation characteristics.
A modified pulsed laser deposition technique was used
to deposit the coatings on the powder materials.9 This technique is distinguished by its ability to make very thin, uniformly distributed, and discrete coatings in particulate systems so that the properties of the core particles can be
suitably modified. An example of a composite particulate
material is shown in Fig. 1. Figure 1 shows that the surface
of the core particle is modified by the attachment of the
secondary nanoparticles. Figure 2 shows a schematic diagram of the system used to fabricate the particulate coatings.
An excimer laser irradiates the target material through the
ultraviolet transparent quartz window. The laser plume,
which is directed perpendicular to the target material, passes
an agitated bed of Y2O2S:Eu31 powder, size approximately
4.5 mm. The thickness and the surface coverage of the coating was controlled primarily by the repetition rate of the
laser and the residence time of the suspension. By controlling the energy as well as the background pressure in the
system, the composition and size of the laser-generated clusters can be controlled.9
Figure 3 shows the cathodoluminescent brightness as a
function of total electron dose for uncoated and TaSi2-coated
Y2O2S:Eu31 phosphor powders. The electron-beam energy
was 2 keV, while a dose rate of 150 mA/cm2 was employed
in the experiments. Figure 3 shows that the CL brightness of
the uncoated samples decreases rapidly with increasing electron dose. In the uncoated Y2O2S:Eu31 powder, the initial
CL degradation is very high, but saturates with higher Coulomb dose. After e-beam irradiation with 15 C/cm2, the
brightness of the sample decreased by 52%. In contrast, the
TaSi2-coated powder exhibits a factor of fourfold decrease in
degradation at the same Coulomb load applied to the uncoated powders. In this case, the total decrease in phosphor
CL brightness is less than 15%, after a 15 C/cm2 beam dose.
These results show the effectiveness of the TaSi2 coating in
a!
Author to whom all correspondence should be addressed. Electronic mail:
rsing@mail.mse.ufl.edu
0003-6951/98/72(15)/1838/2/$15.00
1838
© 1998 American Institute of Physics
Fitz-Gerald et al.
Appl. Phys. Lett., Vol. 72, No. 15, 13 April 1998
FIG. 1. Schematic of a coated phosphor particle to retard the cathodoluminescent degradation.
FIG. 2. Schematic of modified laser deposition system for coating phosphor
powders.
FIG. 3. Comparison of CL degradation rates of the coated and the uncoated
phosphors.
FIG. 5. Initial and final Auger electron spectroscopy spectra obtained from
the TaSi2-coated Y2O2S:Eu31 powders, taken at 0.07 and 15 C/cm2.
retarding CL degradation in oxysulfide powders. To determine the surface composition changes during electron irradiation of the samples, Auger electron spectroscopy ~AES!
experiments were conducted on these samples. Figure 4
shows the AES spectra of the Y2O2S:Eu31 powders before
and after irradiation with an electron dose of 15 C/cm2. Figure 4 shows the characteristic peaks arising from Y, S, and
oxygen from the powder samples, and C contamination at
the surface. The final scan obtained after e-beam irradiation
with 15 C/cm2 shows several new features. First, the carbon
contamination peak disappears, due to an oxygen reaction
with the free carbon during the e-beam irradiation. Second,
the sulfur peak intensity decreases, accompanied by a concomitant shifting in the oxygen peak. This suggests that the
surface is oxidized to Y2O3 :Eu31 during e-beam irradiation.
The formation of a ‘‘dead oxide’’ layer on the surface may
be responsible for the loss in phosphor brightness. The AES
spectra also shows a completely different behavior when the
TaSi2 film is coated on the sample. Figure 5 shows the AES
scan of the TaSi2-coated sample oxysulfide powder before
and after e-beam irradiation with 0.07 and 15 C/cm2. The
TaSi2-coated powders do not show the presence of carbon on
the sample. No significant changes in the spectra is obtained
before and after e-beam irradiation. Thus, no significant decrease in CL brightness occurs in the sample. The AES results tend to suggest that TaSi2 prevents chemical breakdown
and formation of a ‘‘dead layer’’ on the surface of the
sample.
In conclusion, we have demonstrated that TaSi2 coatings
on sulfide-based phosphor powders can significantly increase
CL lifetimes of the phosphor. The TaSi2 coating prevented
chemical transformation of the oxysulfide surface into an
oxide layer.
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1
2
FIG. 4. Initial and final Auger electron spectroscopy spectra obtained from
the uncoated Y2O2S:Eu31 powders, taken at 0.07 and 15 C/cm2.
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